Interview question.

word_count.cpp

#include <vector>
#include <string>
#include <unordered_map>
#include <fstream>
#include <utility>
#include <algorithm>
#include <cassert>
#include <string_view>


std::unordered_map<std::string, size_t> Frequencies(const std::string& ifilename) {
    std::ifstream infile(ifilename);
    assert(infile.good());
    char inchar;
    std::string acc;
    std::unordered_map<std::string, size_t> table;


    infile.get(inchar);
    while (!infile.eof()) {
        while (isalpha(inchar)) {
            acc.push_back(tolower(inchar));
            infile.get(inchar);
        }
        auto search = table.find(acc);
        if (search != table.end()) {
            ++search->second;
        } else {
            table.emplace(acc, 1);
        }
        acc.clear();
        while (infile.get(inchar) && !isalpha(inchar)) {
        }
    }
    return table;
}


void PrintInOrder(const std::unordered_map<std::string, size_t>& frequencies, const std::string& ofilename) {
    std::vector<std::pair<size_t, std::string_view>> ordered_pairs;
    ordered_pairs.reserve(frequencies.size());
    for (auto& [str, freq] : frequencies) {
        ordered_pairs.emplace_back(freq, str);
    }
    std::sort(ordered_pairs.begin(), ordered_pairs.end(),
              [](const std::pair<size_t, std::string_view>& fst,
                 const std::pair<size_t, std::string_view>& snd) {
                  if (fst.first == snd.first) {
                      return fst.second < snd.second;
                  }
                  return fst.first > snd.first;
              });
    std::ofstream outfile(ofilename);
    assert(outfile.good());
    for (auto& [freq, str] : ordered_pairs) {
        outfile << freq << ' ' << str << '\n';
    }
}



int main(int argc, char *argv[]) {
    assert(argc == 3);
    PrintInOrder(Frequencies(argv[1]), argv[2]);
    return 0;
}